In some applications, is it desirable to convert a differential one represented by the difference between two voltages, to a single ended signal, i.e. a varying voltage on a single wire that represents the signal. (There is typically another wire representing ground.) One such application is to convert the differential output of a digital-to-analog converter (DAC) to a single ended signal. One way to accomplish this is by the use of a differential amplifier.
A differential amplifier is a type of electronic amplifier that is well known in the art; it amplifies the difference between two voltages but does not amplify the particular voltages. An operational amplifier, or op-amp, may be used to make a differential amplifier having very high differential-mode gain, very high input impedance, and low output impedance.
A well known example of a differential amplifier made with an op-amp that converts a differential input to a single ended output is shown in FIG. 1. In circuit 100, two inputs V1 and V2 are applied to an op amp 102 through resistors R1 and R2. Op amp 102 provides the amplified difference of the signals V1 and V2 as the output Out, which is fed back to the inverting input of op amp 102 through resistor R3. The non-inverting input of op amp 102 is coupled to ground through resistor R4. The values of R3 and R4 are typically equal to each other. Due to the use of negative feedback (i.e., the feedback of the output to the inverting input of op amp 102), such an op-amp differential amplifier may be made with predictable and stable gain. In some cases, and as is known in the art, certain types of differential amplifiers ma even include several simpler differential amplifiers.
FIG. 2 shows one particular embodiment of such a differential amplifier used to convert the output of a DAC to a single ended signal, that is made by ESS Technology, Inc. In circuit 200, the output of a DAC 202 (indicated by the dashed line) is two output signals V1 and V2. When placed into a larger circuit, DAC 202 acts like an equivalent differential voltage source providing voltages V1 and V2 with output resistances R1 and R2 of 781 ohms (Ω) each, and is thus represented as such in FIG. 2.
As in circuit 100 of FIG. 1, op amp 204 receives the two voltages as inputs and provides the amplified difference as the output Out. Resistor R3 again connects Out to the inverting input of op amp 204, while resistor R4 connects the non-inverting input of op amp 204 to ground. In practice, resistors R3 and R4 again have equal values, and each have resistances of 1330Ω (i.e., 1.33 kilohms or 1.33 kΩ). As will be apparent to one of skill in the art, the gain of the circuit will differ depending upon the value of resistors R3 and R4, and that resistor value may thus be selected to obtain a desired RMS value of Out.
The circuit 200 of FIG. 2 is relatively inexpensive at it requires only a single op amp 204, as well as a few resistors. However, circuit 200 has a limitation in that the output voltages of DAC 202 may change, which in turn causes the inputs to op amp 204, i.e., the voltages that appear on the right hand side of equivalent resistances R1 and R2, to also change with signal level so that they are thus not constant relative to ground. This change in the output voltages of DAC 202 may cause distortion.
It is known that the distortion caused by the varying DAC output voltage change may be reduced by applying as common mode voltage to the inputs to op amp 204, so that the DAC output appears to have an approximately constant voltage regardless of the signal. A typical way to do this is known in the art and is shown in circuit 300 in FIG. 3. Again DAC 302 appears as a differential voltage source with output voltages of V1 and V2, and output resistances R1 and R2 of 781Ω each. Op amp 304 again receives the outputs of DAC 302 and provides an amplified differential voltage at Out. The voltage Out is again fed back to the inverting input of op amp 304 through resistor R3 of 1.33 kΩ, and a similar resistor R4 of 1.33 kΩ again connects the non-inverting input of op amp 304 to ground.
Now, however, the voltages V1 and V2 from DAC 302 are not input directly from resistors R1 and R2 to op amp 304, but rather pass through two additional differential amplifiers that include op amps 306 and 308, and resistors R5 and R6, respectively. The non-inverting inputs of op amps 306 and 308 are connected to a virtual ground Cm, while the inverting inputs receive the voltages V1 and V2 at their respective inverting inputs, and resistors R5 and R6 provide feedback from the outputs of op amps 306 and 308 to their inverting inputs.
The two additional op amps 306 and 308 pass through the difference between each of V1 and V2 and the virtual ground Cm, and thus function as virtual ground current to voltage converters to provide a common mode voltage to op amp 304 along with the differential voltage signal from DAC 302. The outputs of op amps 306 and 308 are passed through resistors R7 and R8 to the inputs of op amp 304, which as in circuit 200 of FIG. 2 provides an open ended signal from the differential signal. Since each input to op amp 304 contains the same common mode voltage, that voltage cancels out and the output Out is again the difference between V1 and V2.
The use of the additional op amps 306 and 308 causes the outputs of DAC 304 to now have a constant common mode voltage level regardless of signal level, which results in less distortion. However, the downside to this configuration is that op amps 306 and 308 typically have the same specifications and operating parameters as op amp 304, and since the op amps are the most expensive part of the circuit (not including DAC 302), the cost of circuit 300 of FIG. 3 is thus about three times the cost of circuit 200 of FIG. 2.
It is thus desirable to find a simple solution that will cause the DAC outputs to have a constant voltage but that is less expensive than the known configuration of FIG. 3 having three expensive op amps.